This is a response to a very specific idea, that Labour suddenly adopting a hardcore europhile policy is an electoral goldmine - and the lib dems are the perfect illustration that it isn't. Other criticism have different responses, i.e Why didn't labour back the tory rebels?
The parliamentary maths relies on every labour MP backing it*. So even if they had, it wouldn't be enough it would just create a minor policy position they'd be held to that might turn out to be the wrong one anyway. Specifically,
EEA amendment - this one I'd be fine with, faux brexit is best brexit but that's still possible anyway and the EU reportedly thinks is the most likely outcome anyway
Parliamentary final vote amendment - seems reasonable enough and I have no idea why the government is against it tbh, MPs won't have choice but the accept whatever deal because the alternative they'll face is a no deal exit
Second referendum amendment - which referendum though? another in/out seems unlikely and a deal/no deal ref has a high risk of a no deal outcome - whatever the deal is will leave much of the country unhappy because this whole mess is ill-conceived and rushed
*not a full list
The problem with the lib dems is that they're comically hypocritical. If Labour had signed a pledge either pre-referendum or pre-2017 election that they will categorically opposite Brexit and then didn't, sure, they'd lose a ton of votes but that didn't happen. Maybe it will alienate a lot of people but this hasn't borne out either at the last election* or in current polling.
*I realise they didn't win the last election, but there hasn't been much data to back their Brexit stance being responsible for it. Depending on which side of the post-mortem you fall, either the country hates Jeremy or Labour put money in the wrong areas (I'm sure you guess where I fall so no need to go into it)